Components of this type would be desirable, for example, as biocompatible and/or biodegradable implant materials. The disadvantage of existing biocompatible implant materials, such as steel or titanium alloys, is that they have a considerably higher modulus of elasticity and density value than bone material. Therefore, the implant may become loose. A further problem of these implant materials is that the metallic implant does not biodegrade by itself in the body, and the implant has to be removed by an operation.
In order to improve removability, known metallic implants also have smooth surfaces. However, these are disadvantageous if bone tissue is to grow into the implant in order to provide improved stabilization of the transition region.
Owing to their low strength compared with metallic implant materials, bioresorbable and/or biodegradable, polymeric implant materials, such as those made from polylactides, can only be used in fields of application where the material is subjected to a small amount of loading.
Therefore, it is an object of the present invention to provide a component which is suitable as biocompatible implant material, has a modulus of elasticity which is adapted to the bone material along with sufficient strength, and to which a porous surface can be imparted, and also a process for producing said component. At the same time, the implant materials should also be bioresorbable and/or biodegradable.
A further object of the invention is to produce a component consisting of magnesium or a magnesium alloy by sintering, this component having a usable strength, such as compressive strength, and a usable modulus of elasticity, so that this component is suitable as biocompatible implant material, for example in the form of screws, pins, bolts, hooks and/or plates.
The object is achieved by a process for producing components consisting of magnesium or magnesium alloys by sintering, in which process
a green compact consisting of magnesium powder and/or a magnesium alloy powder and, if appropriate, an additional alloying constituent is first of all produced,
the green compact is transferred into an inner sintering crucible,
the inner sintering crucible is placed into an outer sintering crucible,
the inner sintering crucible which has been placed in the outer sintering crucible is surrounded with a getter material which is able to bond gases and/or impurities,
the outer sintering crucible with the inner sintering crucible inserted into it and the getter material is heated to sintering temperature, and
the outer sintering crucible with the inner sintering crucible inserted into it and the getter material is allowed to cool after the green compact has been sintered in order to produce the component consisting of magnesium or magnesium alloy.
This object is also achieved by a component which consists of magnesium or magnesium alloy and can be produced by the above process, and also by the use of the component for producing endosseous implant materials which are preferably in the form of a screw, a pin, a bolt, a hook or a plate. The dependent claims specify preferred embodiments.
The starting materials, such as magnesium powder, magnesium alloy powder and/or a powder mixture, are preferably pressed with a pressure of 50 to 125 MPa, and more preferably 75 to 100 MPa.
A decisive factor for successful sintering of the magnesium or of the magnesium alloy or else of a magnesium powder mixture is the use of a getter material. Within the context of the present invention, a getter material is a material which is able to bond gases and/or impurities during the sintering process. The preferred getter material is magnesium, in particular magnesium powder.
The getter material preferably surrounds the inner sintering crucible which has been placed in the outer sintering crucible. With particular preference, the inner sintering crucible which has been placed in the outer sintering crucible is completely enveloped by the getter material. Alternatively, it is possible to select a so-called “labyrinth sintering crucible arrangement”, in which case potential impurities such as oxygen first have to pass completely through the bed of getter material before they reach the inner crucible region. In the case of a “labyrinth sintering crucible arrangement”, it is possible for a plurality of inner sintering crucibles to be arranged one above another, separated by intermediate bases, and these crucibles are preferably covered by a supporting sleeve and held by a retort.
The inner and/or the outer sintering crucible can be made from any material which can withstand sintering temperatures. With particular preference, the inner and/or the outer sintering crucible are produced from steel, preferably unalloyed steel.
The component consists of magnesium or a magnesium alloy. The magnesium alloy is preferably a magnesium-calcium alloy, preferably with a calcium content of up to 1.5% by mass, more preferably 0.2 to 1.0% by mass, most preferably 0.6 to 0.8% by mass. A component which consists of such a magnesium-calcium alloy may be produced, for example, by using magnesium in combination with calcium hydride or magnesium in combination with a magnesium-calcium master alloy as charge materials in an appropriate amount. Examples for the production of a magnesium-calcium alloy (MgCa1) using a calcium hydride or a magnesium-calcium master alloy are as follows:
(a) Mg (98.9%)+CaH2 (1.1%)→MgCa1 (hy)
(b) Mg (98.8%)+MgCa82 (1.2%)→MgCa1 (eu)
(c) Mg (85.5%)+MgCa7 (14.5%) MgCa1 (al)
By reducing the amount of calcium hydride or magnesium-calcium master alloy, magnesium-calcium alloys having a calcium content of less than 1% by mass are obtained.
Components which consist of a magnesium-calcium alloy having a calcium content of up to 1.5% by mass, preferably up to 1% by mass, most preferably up to 0.8% by mass, are preferred to components which consist of pure magnesium. Comparative tests have shown that these alloys have an elasticity and strength which are much improved compared with pure magnesium.
The master alloys which can advantageously be used for producing the components, according to the invention, consisting of magnesium-calcium alloy are the eutectic magnesium-calcium alloys MgCa16 or MgCa82 or a magnesium-calcium alloy having a calcium content of less than 16.2% by mass.
Furthermore, it is also possible to produce other magnesium alloys, such as WE43 (4% yttrium, 3% rare earth elements, remainder magnesium) or W4 (4% yttrium, remainder magnesium), by sintering, as per the process according to the invention. With preference, these alloys according to the invention are also used in medical engineering.
The sintering preferably takes place in a shielding-gas atmosphere or in vacuo. The preferred shielding gas is argon. The sintering preferably takes place at temperatures below the melting temperature of magnesium. The preferred sintering temperature is 600° C. to 642° C. The heating rate from ambient temperature to the sintering temperature is preferably 0.1 to 20 K/min, more preferably 1 to 10 K/min, most preferably 3 to 6 K/min. The sintering preferably lasts for 4 to 64 hours. A further extension to the sintering time can further improve the strength properties of the specimens. The subsequent cooling should preferably take place as quickly as possible, but it is also possible to merely switch off the furnace. However, it is advantageous that the getter material is held under shielding gas or in vacuo for so long that it can no longer ignite.